Problem: Expand.
Explanation: $\begin{aligned} &\phantom{=}(-r^2+3r-7)({r^2-7}) \\\\ &=(-r^2+3r-7)({r^2})+(-r^2+3r-7)({-7}) \end{aligned}$ $\begin{aligned} &=-r^4+3r^3-7r^2+7r^2-21r+49 \\\\ &=-r^4+3r^3-21r+49 \end{aligned}$